The present application is based on and claims priority from Japanese Patent Application 2001-78887, filed Mar. 19, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a DC motor.
2. Description of the Related Art
Usually, a fuel pump mounted in an automotive vehicle is driven by a DC motor. Such a DC motor is operated at a fixed speed or controlled by a speed control unit such as a PWM (pulse width modulation) control system.
Generally, as the voltage applied to the DC motor becomes higher, arc discharge is more likely to occur between the brushes and the commutator of the DC motor. As a result, the brushes and the commutator are subject to wear, and the lifetime thereof extremely reduces.
The PWM control system is comprised of a switching element, which is usually turned on or off at a speed as high as 2 kHz or higher, so that power can be supplied to the armature of the DC motor according to a duty ratio of the turn-on time of PWM signals. However, it is necessary to provide a noise filter to suppress an electro magnetic noise. Such a noise filter is usually bulky and expensive because it includes a coil and a capacitor.
Therefore, the present invention has been made in view of the above problems.
It is a main object of the invention to provide an arc-free DC motor that does not require a noise filter even if controlled by a PWM control system.
According to a feature of the invention, a DC motor includes an armature winding having a first coil group of a plurality of phase coils and a second coil group of a plurality of phase coils, a commutator and a brush unit having two pairs of brushes. In such a DC motor, the first and second coil groups are connected in series to a DC power source via the commutator and the two pairs of brushes to be supplied with motor current. Therefore, the voltage applied to each phase coil is a half as high as the voltage applied to each phase coil of a conventional DC motor that has only a single coil group and a pair of brushes.
As a result, arc discharge is significantly suppressed, so that the lifetime of the brushes can be improved.
According to another feature of the invention, a DC motor includes an armature winding having a first coil group of a plurality of phase coils and a second coil group of a plurality of phase coils, a commutator, and a brush unit including two pairs of brushes, and the first and second coil groups are respectively connected to a DC power source via the commutator and the two pairs of brushes to be alternately supplied with motor current.
Because current is alternately supplied to the first and second coil groups of the DC motor, magnetic noises generated at respective coil groups are cancelled by each other and significantly reduced.
The above DC motor may include a motor current control device that is comprised of a pair of switching elements respectively connected to the first and second coil groups and a PWM current control device for alternately controlling the pair of switching elements.